(1) Field of the Invention
This invention relates to a communication control circuit for sending data to or receiving data from one or more communication nodes connected via a bus in synchronous transfer mode based on unique communication cycles and a communication control method used in such a communication control circuit and, more particularly, to a communication control circuit and communication control method capable of guaranteeing a real-time response to send data.
(2) Description of the Related Art
There has been an increase in the number of devices to be controlled or the amount of data to be transferred in recent factory automation (FA) systems, so high-speed communication lines are required for transferring control commands and data. In addition, to exercise integrated control over units, for example, manufactured by various makers, general-purpose communication lines must be used. On the other hand, the Institute of Electrical and Electronic Engineers (IEEE) 1394 interface is known as a serial interface standard by which a comparatively large amount of data can be transferred (at a rate of 100 to 400 Mbps, for example) mainly between household electronic devices. The use of the IEEE 1394 interface in FA systems is being studied.
In the IEEE 1394 interface, isochronous transfer mode and asynchronous transfer mode can be used as operation modes of data transfer. In the isochronous transfer mode, sending in an isochronous cycle is guaranteed to all of communication nodes that make send requests. The main purpose of the isochronous transfer mode is to transfer image data or voice data in one direction in real time.
The asynchronous transfer mode is an operation mode in which a certain bandwidth is not guaranteed. In the asynchronous transfer mode, data is transferred to a specific node and the node that receives the data must return a response. Therefore, usually control commands and the like are transferred in the asynchronous transfer mode. However, when, for example, a control command for starting a task is sent to a device to be controlled in an FA system, the isochronous transfer mode must be used because real-time control must be guaranteed.
A cyclic data communication system in which in order to send target data, in an asynchronous communication phase a communication node assigned as an ordinary station makes a send request and gets a chance to send in response to a cyclic trigger packet indicative of the beginning of a communication cycle periodically sent from a communication node assigned as a management station in an isochronous communication phase has conventionally been proposed as a related technique (see, for example, Japanese Unexamined Patent Publication No. 2003-8579 (paragraphs [0109]-[0117] and FIG. 9)). In this system, timing with which data is sent is obtained in the isochronous communication phase and the data is transferred in the asynchronous communication phase. This enables control over a high-speed transfer and a high-speed response.
As stated above, real-time data transfer is not guaranteed in asynchronous transfer mode. Accordingly, for example, a control command for starting a task must be transferred in isochronous transfer mode. However, isochronous transfer mode does not entail an obligation to return a response to receive data. To send a response or check that the response is received, control units, such as micro processing units (MPUs), must be located outside communication control large scale integrated circuits (LSIs) at the sending and receiving ends of a control command and communication must be controlled on the basis of processes performed by these external control units. This leads to an increase in the manufacturing costs or installation area of communication systems. In addition, overhead is incurred at the time of transfer by the external control units. For example, in a node at the receiving end, the external control unit must detect the occurrence of a receiving interruption, set response data, or issue a command to begin the transfer of the response data. Such overhead makes it difficult to guarantee real-time data transfer.